Universal mobile telecommunications system (UMTS) is a 3rd generation (3G) asynchronous mobile communication system operating in wideband code division multiple access (WCDMA) based on European systems, global system for mobile communications (GSM) and general packet radio services (GPRS). A long-term evolution (LTE) of UMTS is under discussion by the 3rd generation partnership project (3GPP) that standardized UMTS.
In general, a home eNodeB (HeNB) has a low radio transmission output power compared to the BS owned by mobile communication service providers. Therefore, the service coverage provided by the HeNB is typically smaller than the service coverage provided by an eNB. Due to such characteristics, the cell provided by the HeNB is classified as a femto cell in contrast to a macro cell provided by the eNB from a standpoint of the service coverage. From a standpoint of provided services, when the HeNB provides those services only to a closed subscriber group (CSG), the cell provided by this HeNB is referred to as a CSG cell.
The CSG cell may be defined as a cell broadcasting a CSG indicator set to true and a specific CSG identity (ID). A user equipment (UE) may have a CSG list to which the UE itself belongs as a member thereof, and this CSG list may be changed by a request of the UE or a command of the network. This list is called as a CSG whitelist. Generally, one HeNB may support one CSG.
The HeNB may deliver the CSG ID of the CSG being supported by itself through system information, thereby allowing only the corresponding CSG member UE to be accessed. When a CSG cell is found by the UE, the UE may check which type of CSG is supported by this CSG cell by reading the CSG ID included in the system information. The UE that has read the CSG ID regards the corresponding cell as an accessible cell only if the UE itself is a member of the corresponding CSG cell or the CSG corresponding to the CSG ID is included in the UE's CSG whitelist.
It is not always required for the HeNB to allow the CSG UE to be accessed. Based on a configuration setting of the HeNB, non-CSG member UE may be allowed to be accessed. The type of the UE allowed to be accessed may be changed based on the configuration setting of the HeNB. Here, the configuration setting denotes a setting of the access mode (or may be called as operation mode) of HeNB. The operation mode of HeNB can be divided into three types as follows based on to which type of the UE the HeNB provides a service.
1) Closed access mode: A mode in which services are provided to particular CSG members only. A cell operated by the closed access mode may be called a closed cell. The closed cell may perform access control, which is a process that checks whether a UE is allowed to access and to be granted services in the closed cell.
2) Open access mode: A mode in which services are provided without any restriction of particular CSG members like normal eNB. A HeNB provides a normal cell not a CSG cell. A cell operated by the open access mode may be called an open cell. For clarity, a macro cell is the open cell.
3) Hybrid access mode: A mode in which CSG services are provided to particular CSG members and also services are provided to non-CSG members like a normal cell. A CSG cell operated by the hybrid access mode is called a hybrid cell. The hybrid cell may be defined as a cell broadcasting a CSG indicator set to false and a specific CSG identity. This cell is accessible as a CSG cell by UEs which are members of the CSG cell and as a normal cell by all other UEs. The hybrid cell may check whether a UE is a member or non-member of the hybrid cell. This process may be referred as membership verification. That is, the hybrid cell is considered as an open cell to non-member UEs, while it is considered as a CSG cell to member UEs. Even though member and non-member UEs can access to the hybrid cell, services provided to UEs could be differentiated between the member UEs and the non-member UEs. For instance, the member UEs may be granted with higher data rate than the non-member UEs. Namely, if UEs are connected to the hybrid cell, the hybrid cell would prioritize the member UEs than the non-member UEs.
Furthermore, CSG ID validation may be performed. The CSG ID validation is a process that checks whether the CSG ID received via handover messages is the same as the one broadcast by a target E-UTRAN.
When many UEs try to access to a hybrid cell, overload may occur at a hybrid cell. Accordingly, a method for controlling access to the hybrid cell may be required.